Monitoring Soil Water Content Profiles with a Commercial TDR System : Comparative Field Tests and Laboratory Calibration

The TRIME-FM3 (Imko GmbH, Ettlingen, Germany) time domain reflectometry (TDR) handheld instrument combined with the T3 access tube probe is a commercial soil water content (SWC) profiling system. The main purpose of this study was to evaluate its performance under real field conditions. The evaluation was performed on four sites in Tunisia and France where several measuring campaigns were organized between 2000 and 2002. Reference SWC measurements were also taken systematically. Direct comparisons of the corresponding data showed that a linear correction was generally sufficient to locally calibrate the TRIME (time domain reflectometry with intelligent microelements) measurements. Nevertheless, because of the influence of other interfering factors, a significant error (0.01 < RMSE < 0.07) still remained after this correction. As a possible alternative, the internal standard TRIME-FM3 calibration was also examined. On the basis of our large set of data, new calibration coefficients were derived to convert the TRIME-FM3 internal measurement (a so-called pseudo transit time) into the displayed SWC. However, its general validity should be tested with other independent data. Several laboratory experiments were also performed: TRIME-tube measurements were taken on reference media and on a 10-L sample of soil that was monitored twice during drying from total water saturation to an equilibrium dry state. It was thus possible to quantify the sensitivity of the TRIME-tube measurement to the surrounding medium permittivity. An empirical formula was also established for inferring the soil permittivity from the TRIME-tube measured pseudo transit time.